The demand for traffic capacity, coverage and reliability in the wireless communication systems is seemingly ever-increasing. One bottleneck in the traffic capacity is the limited frequency spectrum available for communication purposes, the limitation being both physical—only part of the frequency spectrum is suitable for communication and the information content per frequency and time is limited, and organizational—the useful part of the spectrum is to be used for a number of purposes including: TV and radio broadcast, non-public communication such as aircraft communication and military communication, and the established systems for public wireless communication such as GSM, third-generation networks (3G), Wireless Local Area Networks (WLAN) etc. Recent development in the area of radio transmission techniques for wireless communication systems show promising results in that the traffic capacity can be dramatically increased as well as offering an increased flexibility with regards to simultaneously handling different and fluctuating capacity needs. Promising techniques are based on the concept of Multiple-Input-Multiple-Output (MIMO) see for example A. Goldsmith et al. “Capacity Limits of MIMO Channels”, IEEE Journal on Selected Areas of Comm., VOL. 21, NO. 5, JUNE 2003. Compared to presently used transmission techniques such as TDMA (as used in GSM) and WCDMA (as used in UMTS), the above exemplified technique represents a much better usage of the available radio frequency spectrum. As an example of the capabilities of, but also the requirement set forth by, the new transmission techniques, the MIMO wireless systems will be briefly described with references to FIG. 1 (prior art). A comprehensive description of the basic principles as well as recent development and areas of research of MIMO is to be found in the above referred article by A. Goldsmith et al.
A radio link in a MIMO system is characterized by that the transmitting end as well as the receiving end may be equipped with multiple antenna elements. The idea behind MIMO is that the signals on the transmit (TX) antennas at one end and the receive (RX) antennas at the other end are “combined” in such a way that the quality (bit-error rate, BER) or the data rate (bits/sec) of the communication for each MIMO user will be improved. Such an advantage can be used to increase both the network's quality of service and the operator's revenues significantly. A core idea in MIMO systems is space-time signal processing in which time (the natural dimension of digital communication data) is complemented with the spatial dimension inherent in the use of multiple spatially distributed antennas. A key feature of MIMO systems is the ability to turn multipath propagation, traditionally regarded as a limiting factor in wireless transmission, into a benefit for the user. MIMO effectively takes advantage of random fading and when available, multipath delay spread, for increasing transfer rates. The prospect of significant improvements in wireless communication performance at no cost of extra spectrum (only hardware and complexity are added) has naturally attracted widespread attention. MIMO is, due to the promising possibilities, considered for enhancing data rates in third generation cellular systems, specifically for the High-Speed Downlink Shared Channel (HS-DSCH).
A compressed digital source in the form of a binary data stream 105 is fed to a transmitting block 110 encompassing the functions of error control coding and (possibly joined with) mapping to complex modulation symbols (quaternary phase-shift keying (QPSK), M-QAM, etc.). The latter produces several separate symbol streams which range from independent to partially redundant to fully redundant. Each is then mapped onto one of the multiple TX antennas 115. Mapping may include linear spatial weighting of the antenna elements or linear antenna space-time precoding. After upward frequency conversion, filtering and amplification, the signals are launched into the wireless channel. N TX antennas 115 are used, and the transmitting block 110 may typically comprise means for N simultaneous transmissions. The symbol streams transmitted on the N TX antennas are commonly referred to as antenna streams. At the receiver, the signals are preferably captured by multiple antennas (M) 120 and demodulation and demapping operations are performed in the receiving block 125 to recover the message. The level of intelligence, complexity, and a priori channel knowledge used in selecting the coding and antenna mapping algorithms will vary substantially depending on the application. This determines the class and performance of the multiantenna solution that is implemented. The MIMO communication may for example occur between a base station (BS) and an user equipment (UE), each provided with the required multiantennas.
The multiplexing alone is, as previously mentioned, not enough for achieving the dramatic increase in gain. Advanced coding/decoding and mapping schemes, i.e. the space-time coding, is essential. A knowledge of the radio channel is needed for the decoding already in today's existing wireless systems such as GSM and UMTS, and in the multiantenna systems this knowledge is absolutely critical. In some of the most promising implementation proposals for MIMO, the knowledge of the channel, is used not only in the demodulation performed in the receiver side, but also in the encoding and modulation on the transmitting side when the system employs adaptive rate control. With adaptive rate control, the transmitter determines a transmission rate appropriate for a given radio channel condition. When the channel condition is good, a high transmission rate is used, whereas when the channel condition is bad, a low transmission rate is used. The transmission rate deter wines the modulation order (e.g., QPSK versus 16QAM) and the coding rate of forward error-correction code (FEC) on the transmitting side. Accurate rate control is highly desirable in that it improves system and user throughput. In WCDMA release 5, transmission rate control is facilitated by a channel quality indicator (CQI) feedback provided by a mobile station. The CQI indicates the receiver signal-to-interference-plus-noise-ratio (SINR) under the current radio condition. In essence, a CQI indicates the highest transmission data rate in order to achieve a certain block error rate (e.g., 10%) under current radio condition. Auxiliary control signaling may be needed to facilitate accurate CQI estimation and rate control in a MIMO system. For example, instantaneous power and code allocation may be signaled from the base station to mobile terminals to facilitate CQI estimation. Since this type of information is signaled to all the mobile terminals in the system, this could be considered as a broadcast control information. Other broadcast control information may also be needed to facilitate accurate CQI estimation. The use of CQI according to WCDMA release 5 is described in 3rd Generation Partnership Project (3GPP), Technical Specification Group Radio Access Networks: Physical channels and mapping of transport channels onto physical channels (FDD), 3GPP TR 25.211, version 5.5.0, September 2003, and in 3rd Generation Partnership Project (3GPP), Technical Specification Group Radio Access Networks: Physical Layer Procedures (FDD) 3GPP TR 25.214, version 5.9.0, June 2003.
In UMTS a common pilot channel (CPICH) is used for the characterization of the dedicated radio channel. First, the receiver relies on the CPICH to obtain an estimate of the channel impulse response that is needed during demodulation. With adaptive rate control, the receiver may also use the CPICH to estimate the highest transmission rate that the current channel condition may support in order to satisfy a targeted block error rate requirement. This transmission rate is then communicated back to the transmitter in a form of channel quality indicator (CQI) per WCDMA release 5. The CPICH is a code channel carrying known modulated symbols scrambled with a cell-specific primary scrambling code. UMTS also provides for secondary CPICHs, which may have individual scrambling codes, which typically are used in operations of narrow antenna beams intended for service provision at places with high traffic density. A similar approach is suggested for MIMO based systems. In MIMO a plurality of common pilot channels (CPICHs), corresponding to the number of transmitting antennas or antenna streams, are used to characterize each of the channels between a transmit antenna and a receive antenna. The requirement for accurate channel characterization in combination with the plurality of CPICHs can make the control signaling relatively extensive, and will take up valuable transmission resources.
Recently, a promising new MIMO technique called PARC (Per-Antenna-Rate-Control) has been proposed for use with HS-DSCH, see S. T. Chung et. Al, “Approaching eigenmode BLAST channel capacity using V-BLAST with rate and power feedback”, Proc. IEEE VTC'01-Fall, Atlantic City, N.J., October 2001. The scheme is based on a combined transmit/receive architecture that performs independent coding of the antenna streams at different rates, followed by the application of successive interference cancellation (SIC) and decoding at the receiver. It requires feedback of the per-antenna rates which are based on the signal-to-interference-plus-noise ratios (SINRs) at each stage of the SIC. With this scheme, it has been shown that the full open-loop capacity of the MIMO flat-fading channel may be achieved, thus offering a potential for very high data rates. SINR feedbacks are already utilized in the link adaptation process employed for HS-DSCH to enhance the spectral efficiency. With link adaptation, the base station selects an appropriate data transmission rate suitable for a given channel condition. Thus, when the channel is in a deep fade, a lower data transmission rate is used, whereas when the channel condition is good, a higher data transmission rate is used. Rate adaptation can also be used to account for the variation of code and power availability. When the base station has lots of available codes and available power, a higher data transmission rate is used. On the other hand, when the base station has only very limited amount of unused codes and power, a lower data transmission rate is used. In a scenario wherein link adaptation is used, all stand-by UEs have to report back to the base station a Channel Quality Indicator (CQI). The CQI is typically a quantized version of the UE receiver SINR, measured, for example, at the output of the receiver. The SINR can be the symbol SINR on a single code of the HS-DSCH, or can be the aggregate SINR summed over all the codes of HS-DSCH.
In the rate adaptation process, an UE, without the knowledge of instantaneous code and power available at the serving base station, typically estimates the output symbol SINR according to a nominal code and power allocation. In SISO operations, nominal code allocations are defined in CQI tables, standardized by the 3GPP, used for rate adaptation, where the nominal power allocation is signaled in one of the downlink control channels. These nominal code and power allocations are established for the purpose of CQI measurement and reporting, and are not intended for reflecting the actual code and power availability at the base station. In fact, the control channel that carries the nominal power allocation has a very slow update rate. The base station receives the SINR feedback and adjusts the reported SINR according to instantaneous power and code allocations that will be allocated to the UE. The adjustment is a linear scaling operation. The adjusted SINR is used by the base station to select an appropriate modulation and coding scheme (MCS).
The scaling process in a MIMO system would be significantly more complex than in the above outlined SISO system. The complexity arises from the plurality of active transmission antennas and the estimated SINR will exhibit convoluted dependence on the power and code allocations. Even a mall error in the adjustment of the SINR will lead to significant degradation of the system throughput. Thus, a correct adjustment of the SINR is of high importance. At the same time it is of interest not to increase the amount of control signaling.